Spectroscopy Technique Advances Spintronics

BERKELEY, Calif., Oct. 16, 2012 — An x-ray spectroscopy technique marks a significant step toward removing a major roadblock impeding the use of certain semiconductors in computing, because processing data using electron “spin” rather than charge would mean smaller, faster and more energy efficient data storage and processing.

Researchers from the US Department of Energy’s Lawrence Berkeley National Laboratory used the method called HARPES (hard x-ray angle-resolved photoemission spectroscopy) to investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor gallium manganese arsenide (GaMnAs). Their findings show that the material’s ferromagnetism arises from both of the mechanisms that have been proposed to explain it.

With the HARPES technique, a beam of hard x-rays flashed on a sample causes photoelectrons from within the bulk to be emitted. Measuring the kinetic energy of these photoelectrons and the angles at which they are ejected reveals much about the sample’s electronic structure. Here the Mn atoms in GaMnAs are shown to be aligned ferromagnetically, with all their atomic magnets pointing the same way. Courtesy of Alex Gray, Stanford and SLAC.
HARPES, based on the photoelectric effect described in 1905 by Albert Einstein, enables scientists to study bulk electronic effects with minimum interference from surface reactions or contamination. It also allows them to probe buried layers and interfaces that are ubiquitous in nanoscale devices and are considered key to smaller logic elements in electronics, novel memory architectures in spintronics, and more efficient energy conversion in photovoltaic cells.

“The key to probing the bulk electronic structure is using hard x-rays, which are x-rays with sufficiently high photon energies to eject photoelectrons from deep beneath the surface of a solid material,” said lead and corresponding author Alexander Gray of Stanford University and the SLAC National Accelerator Laboratory. “High-energy photons impart high kinetic energies to the ejected photoelectrons, enabling them to travel longer distances within the solid. The result is that more of the signal originating from the bulk will be detected by the analyzer.”

Alexander Gray (left) and Charles Fadley at Beamline 9.3.1 of Berkeley Lab’s Advanced Light Source, where they are now carrying out HARPES experiments. Courtesy of Roy Kaltschmidt, Berkeley Lab.
GaMnAs is second only to silicon in widespread use and importance as a semiconductor. If a few percent of the gallium atoms in this semiconductor are replaced with manganese atoms, the result is a dilute magnetic semiconductor. Such materials would be suitable for further development into spintronic devices if the mechanisms behind their ferromagnetism were better understood.

"Right now the temperature at which gallium manganese arsenide operates as a dilute magnetic semiconductor is 170 kelvin," said Charles Fadley, a physics professor at Berkeley Lab’s Materials Science Div. and at the University of California, Davis. "Understanding the actual mechanism by which the magnetic moments of individual manganese atoms are coupled so as to become ferromagnetic is critical to being able to design future materials that would operate at room temperature."

HARPES data on GaMnAs indicate that the ferromagnetism of dilute magnetic semiconductors arises from two distinct mechanisms.
With a better understanding of electronic interactions in dilute magnetic semiconductors, HARPES has provided an important tool for characterizing future materials for faster, smaller and more efficient data storage and processing.

A high-intensity undulator beamline from the Japanese National Institute for Materials Sciences’ Spring8 synchrotron radiation facility in Hyogo, Japan, was used for the experiment. New HARPES studies are now under way at Berkeley Lab’s Advanced Light Source (ALS) using the Multi-Technique Spectrometer/Diffractometer end station at the hard x-ray photoemission beamline.